Hydraulic turbine

ABSTRACT

A hydraulic turbine includes a rotor with a runner, which is concentrically surrounded by a stator, whereby the runner comprises a plurality of runner blades arranged and distributed in a ring around a rotor axis, and each runner blade extends between a runner crown and a runner band; whereby the stator comprises a plurality of guide vanes arranged and distributed in a ring around the rotor axis, and each guide vane extends between an upper stator ring and a lower stator ring; and whereby a predetermined clearance is provided at least between the runner band and the lower stator ring. A substantial reduction of pressure pulsations in the vane-less gap between said runner blades of said runner is achieved by substantially increasing said predetermined clearance.

BACKGROUND OF THE INVENTION

The present invention relates to the technology of hydraulic machines.It refers to a hydraulic turbine.

When designing a hydraulic turbine multiple objectives need to bereached simultaneously. Efficiency appears as the predominant one.However, extending the operating range and ensuring a good behaviour ofthe machine in terms of stability and safety become more and moreimportant. For instance, the customer is now demanding to get a lowvibration level in the turbine pit because high intensity vibrations candamage the machine.

These vibrations result from the mechanical response of the structure tohydraulic excitations coming from the interactions between the rotor(the runner) and stator (RSI).

FIG. 1 shows as an example the main parts of a hydraulic turbine of theFrancis type. The hydraulic turbine 10 of FIG. 1 comprises a verticalrotor with a runner 12 and a turbine shaft 12 a. Runner 12 comprises aplurality of runner blades 16 distributed in a ring around the rotoraxis. Rotor 12, 12 a is concentrically surrounded by a stator 11, whichis arranged between rotor 12, 12 a and a surrounding scroll casing 13.Stator 11 is equipped with a plurality of fixed and movable guide vanes15. Guide vanes 15 direct the stream of water, which is supplied throughscroll casing 13 in a circumferential manner, onto runner blades 15 inorder to put runner 12 into rotational motion. The water leaves runner12 in axial direction through a draft tube 14.

The position of the stator 11 relative to the runner 12 (dashed circlein FIG. 1) is shown in detail in FIG. 2. Guide vane 15 of stator 11extends in vertical direction between an upper stator ring 19 and alower stator ring 20. Runner blades 16 extend in runner 12 between arunner crown 17 and a runner band 18. Between runner band 18 and thesurrounding lower stator ring 20 there is a predetermined clearance C.

Now, the passage of runner blades 16 in front of guide vanes 15, in theso-called vane-less gap VG, produces a local oscillating pressure fieldespecially in case of high head pump turbines (see pressure pulsations22 in FIG. 3). This dynamic pressure field is mainly due to the pressurevariation, on either side of the profiles of guide vanes 15 and runnerblades 16, evolving at different frequencies.

Reducing this oscillating pressure field (or pressure fluctuation level)in the vane-less gap VG is a continuous challenge for the designers andmanufacturers of hydraulic machines, but few techniques exist to raisethis challenge.

Document WO 2008004877 A1 discloses a water turbine of the reactiontype, such as a Francis or Kaplan turbine, comprising a rotor and asurrounding housing, with sealing means between the rotor and thehousing. In this type of turbine there is a problem in that a large partof the losses is connected with seal leakage. Simple labyrinth seals areoften used, but they require large clearances and so have large losses.On this background it is said to be important to provide a water turbineseal system that makes possible much smaller clearances than currentlabyrinth and other conventional seal designs, so as to obtain anefficiency improvement in high head Francis turbines, inter alia. Theproposed sealing means comprises brush seals located at a radialdistance from the rotor axis being a major proportion of the maximumradial dimension of the rotor. The disclosure aims at decreasingdrastically the fluctuation level with an innovative, easy to set up andlow-cost solution.

Document EP 0 565 805 A1 discloses a system for controlling the pulsesof hydraulic pressure and power in a reaction hydraulic turbine. Thesystem comprises means for introducing an additional, controlled,pulsating flow of water at the diffusor elbow, consisting, together oralternatively, of a) at least one moveable body inside the diffusorbetween a first, retracted position and a second, extended position; b)at least one auxiliary duct which is markedly parallel to the diffusorand connected to the said diffusor with its own inlet and outletapertures the inlet opening being positioned at the end of the diffusorelbow controlled by a valve and the outlet opening being positionedadjacent to the piezometric basin; the said moveable body and valvebeing controlled in such a way as to make the volume of the diffusorvary to reduce the pulses of pressure in the diffusor to a minimum andto maintain the flow, the gap, the torque, and the power of the rotor ofthe turbine constant.

Document US 2004/037698 A1 describes an Exit Stay Apparatus for Francisand propeller hydraulic turbines. The purpose of the apparatus is toeliminate the loss of turbine efficiency and strong pulsations in drafttube caused by the axial circular vortex in all turbine operatingregimes other than optimum without a noticeable decrease in maximumefficiency. The Exit Stay Apparatus has a crown and exit stay vanessecured to the crown. When installed in the turbine, the exit stay crownis located immediately after the runner crown, which is truncated at thebottom by a plane perpendicular to the central axis of the turbine. Theexit stay crown together with the truncated runner crown forms waterpassages after the runner blade crown profile exit. The exit stay vanesare arranged in a circular array around the turbine axis, located afterthe runner blades, and attached at the periphery either to the drafttube cone or to an exit stay flange secured to the turbine dischargering and to the draft tube cone.

Document WO 2008/060158 A2 relates to a hydraulic reaction turbinecomprising a runner, a draft tube downstream of the runner, andinjection means for introducing water into the draft tube in order toreduce pressure fluctuations therein, with one or more openings in thedraft tube wall adapted to substantially evenly distribute injectedwater in the axial and/or circumferential direction of the draft tube.

Further examples of hydraulic turbine systems according to the prior artmay be found in the following documents: JP 55060666 which discloses amethod of miniaturising a turbine; JP 55051964 which discloses providingopenings in the turbine to reduce water thrust; JP 06074138 whichdiscloses a system which seeks to reduce friction loss; and U.S. Pat.No. 5,290,148 which is concerned with the thickness of a runner band anda crown, and rigidity in respect of structural matters.

SUMMARY OF THE INVENTION

It is an object of the present invention to decrease in a simple andeffective way the pressure fluctuation level in the vane-less space,i.e. the gap between the runner and the guide vanes.

According to an aspect of the invention, there is provided a hydraulicturbine comprising a runner which may comprise a runner crown, a runnerband and a plurality of runner blades extending between the runner crownand the runner band. The hydraulic turbine may comprise a stator whichin an embodiment surrounds the runner. The stator may comprise an upperstator ring, a lower stator ring and a plurality of guide vanesextending between the upper stator ring and the lower stator ring.

Embodiments of the invention may be characterized in that a clearance isprovided between the runner and the stator, wherein the clearance is inan embodiment arranged to minimize pressure pulsations developing in avane-less gap between the runner blades of the runner and the guidevanes of the stator.

The clearance may be sized such that the local oscillating pressurefield is reduced. The size and location of the clearance may bedependent on the dimensions and shapes of the stator and rotor.

In an embodiment, the clearance may comprise a first clearance betweenthe runner crown and the upper stator ring. A first ratio may be definedbetween the first clearance and the radius of the runner crown. In anembodiment, the first ratio may be in the region of 0.02. Alternatively,the first ratio may be less than or more than 0.02.

In a further embodiment, the clearance may comprise a second clearancebetween the runner band and the lower stator ring. A second ratio may bedefined between the second clearance and the second radius. In anembodiment, the second ratio may be in the region of 0.02.Alternatively, the second ratio may be less than or more than 0.02.

The first ratio may be substantially equal to the second ratio.Alternatively, the ratios may be different.

It is noted that the prior art does not consider the problem of how toreduce the oscillating pressure field. Furthermore, the feature ofproviding a clearance between the runner and the stator, wherein theclearance is arranged to minimize pressure pulsations developing in thevane-less gap, is not disclosed in the prior art.

JP 55060666 provides an example of obtaining and manipulating geometricdata for the inside diameter and width of a runner. In an embodiment ofthe invention, the clearance is larger than found in the prior art. Infact, the teaching of JP 55060666 is to reduce any clearance between therunner and the stator, which would have a detrimental effect on thepressure pulsations. Other modifications are provided in the prior art,but the impact of adjusting the clearance to minimize pressurepulsations is not considered.

According to a second aspect of the invention, there is provided ahydraulic turbine comprising a rotor with a runner, which isconcentrically surrounded by a stator. In an embodiment, the runnercomprises a plurality of runner blades arranged and distributed in aring around a rotor axis. Each runner blade may extend between a runnercrown and a runner band. The stator may comprise a plurality of guidevanes arranged and distributed in a ring around the rotor axis. Eachguide vane may extend between an upper stator ring and a lower statorring. A predetermined clearance may be provided at least between therunner band and the lower stator ring.

An embodiment of the invention may be characterized in that saidpredetermined clearance may be substantially increased with respect tothe prior art in order to dampen pressure pulsations developing in avane-less gap between said runner blades of said runner and said guidevanes of said stator.

According to an embodiment of the invention said runner crown may have afirst radius, said runner band may have a second radius, whereby a firstclearance may be provided between said runner crown and said upperstator ring and a second clearance may be provided between said runnerband and said lower stator ring, wherein the ratio between said firstclearance and said first radius and the ratio between said secondclearance and said second radius may be each higher than 0.02.

Specifically, the ratio between said first clearance and said firstradius may equal the ratio between said second clearance and said secondradius.

According to another embodiment of the invention said runner crown mayhave a first radius, said runner band may have a second radius, wherebya first clearance may be provided between said runner crown and saidupper stator ring and a second clearance may be provided between saidrunner band and said lower stator ring, wherein the ratio between saidfirst clearance and said first radius may be ≦0.02 and the ratio betweensaid second clearance and said second radius may be ≧0.02.

According to a further embodiment of the invention said runner crown mayhave a first radius, said runner band may have a second radius, wherebya first clearance may be provided between said runner crown and saidupper stator ring and a second clearance may be provided between saidrunner band and said lower stator ring, wherein the ratio between saidfirst clearance and said first radius may be ≧0.02 and the ratio betweensaid second clearance and said second radius may be ≦0.02.

It is to be appreciated that one or more of the aspects, embodiments andfeatures of any of the above aspects or embodiments of the invention maybe readily combined, as will be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is to be explained by means of differentembodiments and with reference to the attached drawings:

FIG. 1 shows a typical hydraulic turbine of the Francis type;

FIG. 2 shows as a magnified detail the gap between runner and stator ofthe turbine according to FIG. 1;

FIG. 3 shows the pressure fields and pressure pulsations in the gap ofFIG. 2 for a prior art design;

FIG. 4 shows—in contrast to FIG. 3—the pressure fields and pressurepulsations in the gap of FIG. 2 for an increased gap in accordance withan embodiment of the invention;

FIG. 5 shows various geometrical parameters used for specifying theclearance increase according to an embodiment of the invention; and

FIG. 6 shows an exemplary reduction in pressure pulsation amplitudes dueto the proposed clearance increase.

DETAILED DESCRIPTION OF THE INVENTION

The known solutions to reduce the pressure fluctuations are essentiallyfocused on the design and the shape of components close to the vane-lessgap (VG) area.

Among the most known and identified methods of reducing pressurefluctuations are: changing the blade number and/or the guide vanesnumber; changing the runner blade diameter, the inner diameter of theguide vane and/or the pitch diameter; changing the design of the runnerat the turbine inlet, for example the shape of the leading edge(parabolic shape or linear), the thickness of the blade and/or thecurvature of blades.

Some of these solutions are identified in the following publication:Zhigang et al. Pressure fluctuations in the vane-less space ofhigh-head-pump-turbines—A review. Renewable and Sustainable EnergyReviews. 41 (2015) 965-974.

The above ways have counter effects like hydraulic efficiency decrease,hydraulic instabilities increase and thus a trade-off shall be found toreach minimum pressure pulsation levels.

An embodiment of the present invention significantly increases themechanical clearance between rotor and stator (runner and ring) comparedto prior art standards. Tests performed during two projects for two verydifferent runners have shown impressive results when the clearance isincreased with respect to prior art systems. The increased clearanceacts as a damper for pressure pulsation, reducing the overall level ofpressure pulsation.

One of the major advantages is the ease to set-up this solution, andalso that it has only few drawbacks.

The damping effect of the substantial clearance increase is illustratedin FIGS. 3 and 4.

In FIG. 3, the clearance C1 between runner (runner band 18) and stator(lower stator ring 20) is small in accordance with prior art practice.The relative motion between guide vanes 15 and runner blades 16generates a pressure field with pressure pulsations 22 coming from guidevanes 15 and a respective pressure field coming from the runner blades16 (arrows in FIG. 3(a)). The interaction of both pressure fieldsincreases the pressure (P(t) in FIG. 3(b)), which can be measured bysuitable pressure sensors 21.

Increasing significantly the radial clearance C1->C2 (FIG. 4(a)) has adamping effect as the pulsations can easily escape from the vane-lessgap VG through widened clearance C2. The measured pressure pulsationsare then drastically reduced (P(t) in FIG. 4(b).

A respective clearance increase has been performed during thedevelopment tests of an actual project (nominal head of 305 m) and hasshown impressive results on pressure fluctuations level on the wholerange of head (see FIG. 6), especially at part load (0.5 on x-axis)where the level decreased from 20% to 10%.

It can be seen also on a hill chart of pressure fluctuations that theeffect of the proposed greater clearance C appears not only on the wholerange of head but also especially for medium (50%=130 MW) to low output.This is particularly interesting for applications that specify very lowvalues from partial load to speed no load.

FIG. 5 shows crown clearance CC between upper stator ring 19 and runnercrown 17 and band clearance CB between lower stator ring 20 and runnerband 18. Although shown as equal in FIG. 5, CC and CB may be different.Related to crown clearance CC is the crown radius RC. Related to bandclearance CB is the band radius RB.

In prior art machines a typical CC/RC ratio=CB/RB ratio is about 0.007(<0.012). According to embodiments of the present invention the increasein clearance may be specified to be CC/RC ratio=CB/RB ratio>0.02 (tested0.0224 and 0.0298).

However, both ratios CC/RC and CB/RB need not be equal, but may differ,while both ratios are higher than 0.02 (CC/RC≠CB/RB and CC/RC>0.02 andCB/RB>0.02).

Furthermore, the ratio CC/RC between said first clearance CC and saidfirst radius RC may be ≦0.02, while the ratio CB/RB between said secondclearance CB and said second radius RB may be ≧0.02.

Alternatively, the ratio CC/RC between said first clearance CC and saidfirst radius RC may be ≧0.02, while the ratio CB/RB between said secondclearance CB and said second radius RB may be ≦0.02.

Advantages of embodiments of the present invention include: The proposedsolution can be generally applied to hydraulic pumps/turbines. It canalso be applied to Francis turbines (mainly for high head machines). Itgives an immediate gain without additional costs. It is easy to set up.There is no particular constraint as it is directly taken into accountin the mechanical design. For an installed base there is only toincrease the clearance gap if this is enough (fast and cheaper). For newprojects: There is an additional and positive effect for a designdedicated on the decrease of pressure fluctuation level in the vane-lessgap.

This written description uses examples to disclose the invention,including the preferred embodiments, and also to enable any personskilled in the art to practice the invention, including making and usingany devices or systems and performing any incorporated methods. Thepatentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal languages of the claims.

What we claim is:
 1. Hydraulic turbine comprising: a runner comprising arunner crown, a runner band and a plurality of runner blades extendingbetween the runner crown and the runner band; and a stator whichsurrounds the runner, the stator comprising an upper stator ring, alower stator ring and a plurality of guide vanes extending between theupper stator ring and the lower stator ring, wherein a clearance betweenthe runner and the stator is arranged to minimize pressure pulsationsdeveloping in a vane-less gap between the runner blades of the runnerand the guide vanes of the stator.
 2. The hydraulic turbine as claimedin claim 1, wherein the clearance comprises: a first clearance betweenthe runner crown and the upper stator ring, and a first ratio is definedbetween the first clearance and the radius of the runner crown.
 3. Thehydraulic turbine as claimed in claim 2, wherein the first ratio is0.02.
 4. The hydraulic turbine as claimed in claim 2, wherein the firstratio between said first clearance and said first radius is less than0.02.
 5. The hydraulic turbine as claimed in claim 2, wherein the firstratio between said first clearance and said first radius is greater than0.02.
 6. The hydraulic turbine as claimed in claim 2, wherein theclearance comprises: a second clearance between the runner band and thelower stator ring, and a second ratio is defined between the secondclearance and the second radius.
 7. The hydraulic turbine as claimed inclaim 6, wherein the second ratio between said second clearance and saidsecond radius is 0.02.
 8. The hydraulic turbine as claimed in claim 6,wherein the second ratio is less than 0.02.
 9. The hydraulic turbine asclaimed in claim 6, wherein the second ratio is greater than 0.02. 10.The hydraulic turbine as claimed in claim 6, wherein the first ratiobetween the first clearance and the first radius substantially equalsthe second ratio between the second clearance and said second radius.